The present invention relates to an improved racket frame, and more particularly relates to improvement in the ball-striking characteristics of a racket frame used for ball games such as tennis.
A racket frame of this type has a general construction in which a head is connected to a grip via a shaft including a throat the head having a substantially oval shape defining a face formed by a latticework of strings held in tension. The head is made up of a crown and shoulders.
When a ball is struck by such a racket frame, the ball and the racket frame vibrate in different modes of vibration soon after contact of the ball with the face of the racket frame.
In the case of most conventional racket frames, the thickness of the frame, i.e. the dimension of the racket as measured in a direction perpendicular to the plane of the face is about 18 mm or less and is uniform over the entire length of the frame. This small thickness attenuates abrupt decay of vibration after the racket face has struck a ball. In addition, the racket frame is usually designed to have a natural frequency in a range from 90 to 140 Hz. Such a low natural frequency of the racket frame causes a big difference in the vibratory performance between the ball and the racket frame and, as a consequence, the vibratory energy of the racket frame cannot be efficiently utilized for to accelerate the ball by repulsion.
In an attempt to overcome these disadvantages, a new racket frame is proposed in U.S. Pat. No. 4,664,380. In one embodiment of this prior proposal, the thickness of the racket frame increases gradually from the joint of the shaft with the grip, reaches the largest value about the middle of the length of the head and decreases gradually towards the head end. By so varying the thickness along the longitudinal direction of the racket frame, it is intended to make the natural frequency of the racket frame closer to the excitation frequency of balls.
With such a construction of the prior proposal, however, vibration of the racket frame abruptly decays when a ball is struck by a section of the face close to the head end due to the reduced thickness of the frame in that area. Such abrupt attenuation in vibration results in a low flying speed of the ball and poor accuracy in striking. Thus, the ball cannot fly a long distance in an intended direction.